The present invention is related to a cleaning device applicable to an electrostatic recording system, such as for instance a copying or a printing system, in order to remove residual developer from the surface of an image-forming member.
In a typical printing or copying process, a charged latent image is formed on an image-forming member by image-wise exposure. The image-forming member can be an endless member such as a drum or a belt. Typical graphical processes include amongst others magnetography, ionography and electrography, particularly electrophotography. In the latter process for instance, the charged latent image is formed on a pre-charged photosensitive member by image-wise exposure to light. The latent image is subsequently made visible on the image-forming member with developer at a development zone, the developer comprising, or consisting of, charged toner. After the development of the latent image, the developed image is transferred to a recording medium, directly or via one or more intermediate image-carrying members, where it may be permanently fixed. Examples of intermediate image-carrying members are endless belts. In practice the transfer from an image-delivering member being either an image-forming member or an intermediate image-carrying member to an image-receiving member being either an intermediate image-carrying member or a recording medium may be incomplete. Multiple subsequent transfers are possible. In normal operating conditions, typical transfer efficiencies range from 95% to 100%. The residual image on the image-delivering member has to be removed because otherwise the image quality of subsequently formed or transferred images can be seriously disturbed.
This residual image has to be removed before re-entering into the development zone. Otherwise this could lead to serious image defects because of mixing up of the new developed or transferred image with the residual image.
This cleaning action is executed by a cleaning station positioned downstream from the transfer zone. The cleaning station comprises at least a revolving brush which can be engaged against the image-delivering member for removing residual developer therefrom, a high voltage collecting roller in rolling contact with the brush roller for brush de-toning and a scraper blade contacting the high voltage roller for scraping developer therefrom.
The cleaning of the high voltage roller is a problem. This roller is a rigid roller in rolling contact with the cleaning brush. In the contact zone, developer is transferred to the high voltage roller by biasing the high voltage roller such that an attractive electrical field is created. A cleaning blade is positioned downstream of the contact zone to scrape off the developer from the high voltage roller. Usually to maximize force, the cleaning blade is positioned at an obtuse contact angle. The contact angle is defined with respect to a line tangent to the point of contact of the cleaning blade with the rotating high voltage roller and is the angle between this tangent line, at the uncleaned section of the roller, and the cleaning blade. This obtuse contact angle is typically between 160 and 170 degrees. The cleaning blades used as such are usually very stiff and rigid amongst others to prevent flip over of the cleaning blade as for instance when there is no developer on the roller. As a consequence, more elastic cleaning blades are unsuited because such flip over is detrimental both with respect to the lifetime of the blade and the cleaning efficiency. A cleaning blade mounted at an obtuse contact angle is typically made of an incompressible rigid material such as stainless steel.
In U.S. Pat. No. 4870466 (lida, assigned to Ricoh) a cleaning blade is disclosed which is mounted at an acute contact angle, i.e. a trailing cleaning blade, with respect to the high voltage roller. However, the contact angle disclosed seems to be clearly smaller than 45 degrees. As a result, the contact area between the cleaning blade and the high voltage roller is rather large. It is found that cleaning at such small angles is inefficient. Moreover, the cleaning blade is mounted such that the waste toner which is removed from the high voltage roller can not fall down freely to be further removed, but instead, at least to some extent, will build up between the roller and the cleaning blade and as such may even push the cleaning blade away from the high voltage roller.
It is an object of the present invention to provide a cleaning blade to scrape off developer and debris from the surface of a collecting roller, being used as a brush de-toning device in a cleaning unit.
It is a preferred object of the invention to mount the cleaning blade such that the cleaning blade has a good cleaning ability for an extended period of time and that the waste developer which is scraped off the high voltage roller is allowed to freely fall down to be further removed by a revolving auger and/or an air flow.
It is a further preferred object of the invention to provide a compressible cleaning blade, which is not damaged when exposed to carrier particles and other debris, which may be present on the surface of the high voltage roller.
It is still a further preferred object of the invention to provide a wear-resistant cleaning blade and an associated mounting position which allows for an efficient cleaning of a collecting roller having a fairly rough surface, i.e. with Ra ranging from 0.05 to 0.15.
According to a first aspect of the invention there is provided a cleaning device being part of a copying or printing system for removing residual developer from the surface of an image-delivering member comprising:
a cleaning brush in rolling contact with said surface of said image-delivering member for removing residual developer therefrom, said cleaning brush being rotatable in a first predetermined direction;
a collecting roller in rolling contact with said revolving brush, said collecting roller being rotatable in a second predetermined direction;
means for biasing said collecting roller to generate an electrical field which attracts the residual developer from the cleaning brush and collects it onto its surface; and
a cleaning blade having a forward end portion in frictional contact with the collecting roller wherein, in an operative orientation of the device, said cleaning blade is in contact with said collecting roller at a contact position where the collecting roller is moving in an upward direction, said cleaning blade being mounted such that the contact angle (as hereinbefore defined) is less than 90 degrees.
In an embodiment of the invention, the cleaning device is retractable. The cleaning device is a part of a copying or printing system and is intended for removing residual developer from the surface of an image-delivering member such as for instance an image-forming member or an image-carrying member. Examples of image-forming members are drums or belts with a photoreceptive or a magneto-sensitive outer layer. Examples of image carrying members are seamed or seamless intermediate transfer belts. Such an intermediate transfer belt may be composed of an electrically semi-insulating or insulating material with a low surface energy, or comprises at least a top coating of such a material. Examples of such a material are polyesters such as e.g. Hytrel 7246, polyimides, polycarbonates or dissipative polymer blends.
The collecting roller in rolling contact with said revolving brush is electrically biased such that an electrical field is generated which attracts the residual developer from the cleaning brush and collects it onto its surface.
The cleaning blade is preferably composed of an elastic material with a hardness ranging from 50 to 80 Shore A.
We are aware that cleaning blades are widely used, particularly for cleaning the image-forming member. In electrophotography the image-forming member is usually a drum or a belt covered with an organic photo-conductive layer. The cleaning of this smooth sensitive layer can however in no way be compared with the cleaning of the high voltage roller, which is usually an incompressible rigid roller. Irrespective of the positioning of the cleaning blade, the force required to completely remove the residual developer from the image-forming layer is such that the image-forming layer is damaged. In practice, the cleaning blade is usually mounted such that the image-forming member is not damaged which by consequence results in an incomplete cleaning. Therefore, extra cleaning means are provided, e.g. in the form of a revolving cleaning brush in rolling contact with the image-forming members, to improve cleaning results.
The developer used in the recording system with which the cleaning device according to the invention is associated can be a mono-component or a two-component developer. A common development technique uses a two-component developer material of toner particles adhering tribo-electrically to larger carrier beads. When the developer material, contained in a developer unit, is placed in an appropriate magnetic field, the carrier beads with the toner thereon form a magnetic brush. As the carrier beads and the toner particles are oppositely charged, in the development zone the toner particles are attracted from the carrier beads to develop the latent image on the image-forming member. In case of a two-component developer it is clear that both the developed image and the residual image are primarily composed of toner particles. However, due to failures, as e.g. wrong sign carrier beads, very small numbers of carrier beads may be transferred to the image-forming member in the development zone and subsequently picked up by the cleaning brush and thereafter collected on the collecting roller. Contrary to e.g. a metal cleaning blade mounted at an obtuse contact angle, the cleaning blade of the present invention easily removes such hard carrier beads without causing damage to the blade.
The cleaning brush rotates in a first predetermined direction, which is preferably opposite to the propagation direction of the image-delivering member. The collecting roller contacts the cleaning brush and rotates in a second predetermined direction, preferably opposite to said first predetermined direction. The collecting roller may be a freely rotating roller or may be driven. The cleaning brush and the collecting roller may be independently driven and their rotation speed may be independently controlled. The collecting roller is incompressible and electrically conductive and bias means are provided to apply a voltage to the collecting roller in order to create an electrical field which is attractive for the developer gathered on the cleaning brush.
The cleaning blade according to the present invention is preferably an elastic cleaning blade with a hardness in the range from 50 to 85 Shore A. The rebound resilience is typically in the range from 20 to 40%. Preferably a polyurethane cleaning blade is used. The cleaning blade is mounted at an acute contact angle. The thickness of the cleaning blade is typically between 1.5 mm and 4 mm. The cleaning blade is partly attached to a support such that the free portion of the cleaning blade has a length typically in the range from 4 to 11 mm. The free portion of the blade is the portion which is not attached to the support. The blade material is compressible and the thickness and free length of the blade are chosen such that at least the forward end portion of the blade is allowed to bend slightly while in contact with the collecting roller, i.e. while exerting pressure on the blade. Particularly the blade is positioned such that the pressure exerted by the blade on the collecting roller would correspond to an impression of the blade in the incompressible collecting roller ranging from 0.25 mm to 1 mm. The cleaning blade is mounted such that it contacts the collecting roller at a position where the collecting roller moves in an upward direction. This enables waste developer being scraped off the collecting roller to freely fall down and inhibits potential build up of waste material between the blade and the collecting roller.
In an embodiment of the invention, the cleaning blade is mounted such that the acute contact angle of the cleaning blade with respect to the collecting roller is in the range from 60 to 80 degrees. It has been observed that smaller contact angles result in inefficient cleaning, more particularly, a contact angle below 60 degrees causes developer filming on the collecting roller which results in a decreased de-toning ability of the collecting roller and consequently in a less efficient cleaning of the image-delivering member. Moreover, a blade mounted at such a small contact angle is not able to remove carrier beads from the surface of the collecting roller. In an embodiment of the invention, the collecting roller is composed of a metal. Particularly, aluminum or steel can be used. In such case, the surface of the collecting roller may be hard anodized to increase at least the hardness of the roller. Alternatively, a ceramic coating may be provided as a surface layer. Particularly the surface can have an average roughness, Ra, in the range from 0.05 to 0.15.
In an embodiment of the invention, the cleaning device further comprises an auger, being positioned below the collecting roller to remove the waste developer, which is scraped off the collecting roller by the cleaning blade. An air flow may be provided to assist in the removal of the waste developer. Alternatively, instead of an auger, only an air flow may be provided to remove the waste.
According to another aspect of the invention, there is provided a method for cleaning a surface of an image-delivering member, which is a part of a copying or printing system, the method comprising the steps of:
contacting the outer surface of said image-delivering member with a cleaning brush to remove residual developer therefrom, said cleaning brush rotating in a first predetermined direction;
establishing a rolling contact between said revolving brush and a collecting roller rotating in a second predetermined direction,
biasing said collecting roller such that an electrical field is generated which attracts said residual developer from said cleaning brush and collects it onto its surface; and
scraping off said collected residual developer from said surface of said collecting roller with a cleaning blade, said cleaning blade having a forward end portion in frictional contact with the collecting roller at a contact position where the collecting roller is moving in an upward direction, said cleaning blade being mounted such that the contact angle (as hereinbefore defined) is less than 90 degrees.